Rollup merge of rust-lang#59139 - oli-obk:unions_are_a_good_thing, r=RalfJung

Unregress using scalar unions in constants.

fixes rust-lang#59126

r? @RalfJung

I also made a bunch of these invariants panic instead of report an error.

Author: kennytm

src/librustc_mir/const_eval.rs

@@ -65,12 +65,12 @@ pub(crate) fn eval_promoted<'a, 'mir, 'tcx>(
 fn mplace_to_const<'tcx>(
     ecx: &CompileTimeEvalContext<'_, '_, 'tcx>,
     mplace: MPlaceTy<'tcx>,
-) -> EvalResult<'tcx, ty::Const<'tcx>> {
+) -> ty::Const<'tcx> {
     let MemPlace { ptr, align, meta } = *mplace;
     // extract alloc-offset pair
     assert!(meta.is_none());
-    let ptr = ptr.to_ptr()?;
-    let alloc = ecx.memory.get(ptr.alloc_id)?;
+    let ptr = ptr.to_ptr().unwrap();
+    let alloc = ecx.memory.get(ptr.alloc_id).unwrap();
     assert!(alloc.align >= align);
     assert!(alloc.bytes.len() as u64 - ptr.offset.bytes() >= mplace.layout.size.bytes());
     let mut alloc = alloc.clone();
@@ -79,16 +79,16 @@ fn mplace_to_const<'tcx>(
     // interned this?  I thought that is the entire point of that `FinishStatic` stuff?
     let alloc = ecx.tcx.intern_const_alloc(alloc);
     let val = ConstValue::ByRef(ptr, alloc);
-    Ok(ty::Const { val, ty: mplace.layout.ty })
+    ty::Const { val, ty: mplace.layout.ty }
 }
 
 fn op_to_const<'tcx>(
     ecx: &CompileTimeEvalContext<'_, '_, 'tcx>,
     op: OpTy<'tcx>,
-) -> EvalResult<'tcx, ty::Const<'tcx>> {
-    // We do not normalize just any data.  Only scalar layout and slices.
+) -> ty::Const<'tcx> {
+    // We do not normalize just any data.  Only non-union scalars and slices.
     let normalize = match op.layout.abi {
-        layout::Abi::Scalar(..) => true,
+        layout::Abi::Scalar(..) => op.layout.ty.ty_adt_def().map_or(true, |adt| !adt.is_union()),
         layout::Abi::ScalarPair(..) => op.layout.ty.is_slice(),
         _ => false,
     };
@@ -100,11 +100,11 @@ fn op_to_const<'tcx>(
     let val = match normalized_op {
         Ok(mplace) => return mplace_to_const(ecx, mplace),
         Err(Immediate::Scalar(x)) =>
-            ConstValue::Scalar(x.not_undef()?),
+            ConstValue::Scalar(x.not_undef().unwrap()),
         Err(Immediate::ScalarPair(a, b)) =>
-            ConstValue::Slice(a.not_undef()?, b.to_usize(ecx)?),
+            ConstValue::Slice(a.not_undef().unwrap(), b.to_usize(ecx).unwrap()),
     };
-    Ok(ty::Const { val, ty: op.layout.ty })
+    ty::Const { val, ty: op.layout.ty }
 }
 
 fn eval_body_and_ecx<'a, 'mir, 'tcx>(
@@ -488,7 +488,7 @@ pub fn const_field<'a, 'tcx>(
     let field = ecx.operand_field(down, field.index() as u64).unwrap();
     // and finally move back to the const world, always normalizing because
     // this is not called for statics.
-    op_to_const(&ecx, field).unwrap()
+    op_to_const(&ecx, field)
 }
 
 // this function uses `unwrap` copiously, because an already validated constant must have valid
@@ -534,9 +534,9 @@ fn validate_and_turn_into_const<'a, 'tcx>(
         // Now that we validated, turn this into a proper constant.
         let def_id = cid.instance.def.def_id();
         if tcx.is_static(def_id).is_some() || cid.promoted.is_some() {
-            mplace_to_const(&ecx, mplace)
+            Ok(mplace_to_const(&ecx, mplace))
         } else {
-            op_to_const(&ecx, mplace.into())
+            Ok(op_to_const(&ecx, mplace.into()))
         }
     })();
 

src/test/ui/consts/union_constant.rs

@@ -0,0 +1,11 @@
+// compile-pass
+
+union Uninit {
+    _never_use: *const u8,
+    uninit: (),
+}
+
+const UNINIT: Uninit = Uninit { uninit: () };
+
+fn main() {}
+